1520s, "electric ray" (flat fish that produces an electric charge to stun prey or for defense), from Latin torpedo "electric ray," originally "numbness, sluggishness" (the fish so called from the effect of being jolted by the ray's electric discharges), from torpere "be numb" (from PIE root *ster- (1) "stiff"). The sense of "explosive device used to blow up enemy ships" is first recorded 1776, as a floating mine; the self-propelled version is from c. 1900. Related: Torpedic.
Torpedo. A fish which while alive, if touched even with a long stick, benumbs the hand that so touches it, but when dead is eaten safely. [Johnson]